1. Field of the Invention
The present invention relates to an improved rotary drum filter.
2. Problem to be Solved
Rotary drum filters have been utilized for dewatering slurries for chemical, food, minerals beneficiation, pulp and paper, pharmaceutical and other industrial plants. They have also been used in waste water treatment plants to dewater industrial and municipal sludges, and to thicken lime sludge.
The filtration area on the drum surface is typically divided into independent cells connected by pipes to a vacuum system. The drum deck is formed by replaceable filter grits which support the filter medium. The filter medium is typically a cloth fabricated from fabric or metal. A vacuum pump provides the forces necessary to accomplish dewatering. The vacuum in effect sucks the slurry liquor through the filter medium. The solids in the feed slurry contained in the filter vat form a cake on the filter cloth. Feed slurry is introduced continuously into the filter vat and an adjustable overflow controls the liquid level. The vat is equipped with an agitator to keep the solids from thickening up and settling, processes which will interfere with the efficient operation of the filter. Discharge of the cake takes place on the descending side of the drum.
When used for thickening lime sludge, the filter drum is partly immersed in a vat containing lime sludge. At the beginning of the filtering process, a precoat filter layer of lime sludge is formed on the surface of the filter drum. A scraper is used for scraping off the dried lime sludge from the surface of the precoat. The dried lime sludge falls onto a conveyer and further on to a feed screw which introduces the dried lime sludge into a lime sludge reburning kiln. After the scraper scrapes off the dried lime sludge, the whole precoat is removed and a new precoat formed. Typically, the precoat is removed by directing high pressure liquid jets against the precoat under the scraper. The liquid jets are oriented in a manner such that the liquid jets break the precoat and drop it into the vat, and clean the filter cloth. Typically, the liquid jets emanate from nozzles that are continuously reciprocated in a dimension parallel to the axis of rotation of the drum.
However, the process of removing the precoat from the drum filter in the manner described above requires an inordinate quantity of water which degrades the efficiency of conventional rotary drum filters and increases operating costs of such systems. It has been found that such inordinate amounts of water are required to remove the precoat because the filter cake on the descending side of the drum filter is thinner, since most of the liquid has been extracted by the vacuum within the drum, and the filter cake on the portion of the descending side of the drum is compacted due to contact with the underside of the scraper. Furthermore, the precoat layer on descending side of the filter drum has a solids content of about 65% to about 78%. It has been found that these factors make it difficult to remove the precoat from the descending side of the drum which is under the scraper.
It is also been found that conventional devices for reciprocating the nozzles in a dimension parallel to the rotational axis of the drum are typically bulky and inefficient due to the fact that, in addition to the nozzles, associated components such as electronic circuitry, wiring, hoses, etc. must travel the full length of the drum thereby causing extreme wear of the components which produce the reciprocating motion. Typically, these components which produce the reciprocating motion include a chain and sprocket drive mechanism. Such wear causes frequent breakdowns of the reciprocating device. Furthermore, it has been found that the hoses frequently become entangled with other components associated with the drum filter while the hoses are traveling along the longitudinal length of drum.
Another problem with conventional rotary drum filters is the loss of vacuum resulting when the precoat is removed from the drum and the filter cloth is exposed. Conventional filters provide shut off valves which deactivate the vacuum in the cell of the drum corresponding to the exposed filter cloth. Furthermore, since the drum is continuously rotating, the interior cell corresponding to the exposed filter cloth must have the vacuum within that cell deactivated once the filter cake is removed, and then activated once new lime sludge covers the surface of the drum corresponding to the cell in question. However, the addition of these shut off valves adds to the operational complexity of the rotary drum filter and increases manufacturing costs of such systems.
Bearing in mind the problems and deficiencies of conventional rotary drum filters, it is an object of the present invention to provide a new and improved rotary drum filter that utilizes less water in the operation thereof.
It is another object of the present invention to provide a new and improved rotary drum filter in which the pressure of the liquid jets directed toward the drum filter surface is less than that in conventional rotary drum filters.
It is a further object of the present invention to provide a new and improved rotary drum filter wherein the vacuum within the drum interior may be constantly maintained in all portions or cells of the drum interior.
It is yet another object of the present invention to provide a reciprocating mechanism for reciprocating nozzles in a dimension parallel to the drum rotational axis that is lightweight, structurally stable and inexpensive to manufacture.
A characteristic feature of the present invention is that liquid jets are directed against the precoat and outer layers located on the ascending side of the drum filter.
Another characteristic feature of the present invention is that a strip of precoat and outer layer is dispersed on the ascending side of the drum and then immediately refilled with lime cake wash water so as to minimize vacuum loss within the corresponding interior cell of the drum.
Another characteristic feature of the apparatus of the present invention is that a reciprocating mechanism is provided that reciprocates a pair of nozzles in a dimension substantially parallel to the drum rotational axis wherein each nozzle travels half the longitudinal distance of the drum.
An advantage of the rotary drum filter of the present invention is that less water is utilized in the operation thereof.
Another advantage of the rotary drum filter of the present invention is that vacuum loss within the interior of the drum is minimized during the operation thereof.
A further advantage of the rotary drum filter of the present invention is that the pressure of liquid jets directed against the drum filter is significantly lower than in conventional rotary drum filters.
Another advantage of the present invention is that reciprocating mechanism utilized therein reciprocates a minimum number of components with the nozzles.